The invention relates to an injection-locked oscillator and, in particular, to an injection-locked oscillator that utilises a pulse generator to maintain a phase-alignment between a reference signal and the signal output by the oscillator.
A phase-locked loop is a well known circuit for generating signals having a predetermined frequency relationship with a reference signal. In its most basic form, a phase-locked loop comprises an oscillator that is controlled by means of a feedback loop. The feedback loop takes the output of the oscillator, compares it with a reference signal and adjusts the oscillator accordingly. Typically the feedback loop comprises a divider for dividing the output signal, a phase comparator for comparing the phase of the divided signal with the reference signal and a charge pump for outputting a pulse of charge that either speeds up or slows down the oscillator, in dependence on the phase comparison. The phase-locked loop will also typically include a filter for removing spurious noise from the charge pulse before it reaches the oscillator input.
By incorporating a divider into the feedback loop, the phase-locked loop is able to output frequencies that are an integer multiple of the reference signal. Some phase-locked loops are able to output signals that are non-integer multiples of the reference signal. These phase-locked loops are known as “frac-N” phase-locked loops. Frac-N phase-locked loops comprise a fractional divider in the feedback loop. The fractional divider divides the output signal by a varying integer to achieve an output signal that is a non-integer multiple of the reference signal.
Although phase-locked loops are effective at generating signals having a wide range of different frequencies from a single frequency reference, they are not always suitable for low-power implementations. The phase comparator and the divider tend to be particularly power-hungry. Therefore, for some implementations it is preferred to use a different way of generating signals, such as injection locking.
Injection locking is a physical phenomenon whereby an oscillator synchronises with an external periodic signal when the frequency of that signal is sufficiently close to the natural frequency of the oscillator or one of the harmonics of that natural frequency. Suppose a signal of frequency ωi is injected into an oscillator having a free-running frequency of ω0. When ωi and ω0 are quite different, the oscillator outputs a signal containing beats of the two frequencies. However, as ωi approaches ω0 the beat frequency decreases until, as ωi enters what is known as the “locking range”, the beats disappear and the oscillator starts to oscillate at ωi. Injection locking also happens when ωi is close to a harmonic or sub-harmonic of ω0, i.e. nω0 or 1/n ω0. Locking the oscillator to a harmonic of ω0 can be used for frequency division while locking the oscillator to a sub-harmonic of ω0 can be used for frequency multiplication.
In one implementation of an injection-locked oscillator a stream of narrow pulses is injected into the oscillator. This is shown generally in FIG. 1. The pulses may help to keep the oscillator synchronised with the injection frequency by periodically shorting the tank of the oscillator. An example of such a circuit is shown in FIG. 2, which shows a pulse generator 201 connected to an oscillator 202, which is in turn connected to an output buffer 203. The stream of pulses generated by the pulse generator control a switch 204 that shorts the capacitor bank of the oscillator when the pulse goes high. The pulse train frequency is set to a multiple or sub-multiple of the desired frequency, depending on whether frequency division or multiplication is required. The width of the pulses should preferably be set to be much less than the period of oscillation of the reference signal so that the quality factor Q of the tank is not too severely degraded.
FIG. 3 shows examples of the signals involved. The reference signal is shown at 301 with the stream of pulses output by the pulse generator shown at 302. As shown in the figure, each pulse corresponds to a rising edge of the reference signal. If the pulse occurs at the zero crossing of the oscillator signal (signal 305) it will have no effect on the output (assuming that the pulse is very narrow). If the shorting occurs either before or after the zero crossing of the oscillator signal, the oscillator signal is pushed towards the zero crossing point of the reference signal so that its own phase is either delayed (signal 303) or advanced (signal 304). The effect of the pulse is therefore to increase the frequency of the oscillator signal if it is late relative to the reference signal and to decrease the frequency of the oscillator signal if it is early relative to the reference signal. This periodic phase-shifting causes the average frequency of the oscillator to match the desired frequency.
Injection-locked oscillators are a compact, low power and elegant solution to replace a phase-locked loop. However, injection-locked oscillators have some major drawbacks because, by definition, if the oscillator signal is injection-locked to the reference signal it means that the phase of the oscillator signal sampled at the rising edge of the reference signal is constant. Therefore, the injection-locked oscillator is not capable of non-integer frequency multiplication or phase modulation.
Therefore, there is a need for an improved oscillator that combines the advantages of an injection-locked oscillator with the capability to perform non-integer frequency multiplication and phase modulation.